1. Field of Invention
The present invention relates to a process for manufacturing toothed belts of elastomeric material, comprising the following steps: making a semifinished product in the form of a continuous strip; wrapping the semifinished product in the form of a strip over at least two wrapping rollers having parallel axes and being spaced apart according to a predetermined distance between centers, said wrapping being carried out according to a wrapping pitch corresponding to the width of the semifinished product in order to achieve an annular element of the desired circumferential extension or length, of at least the same width or axial length as the toothed belt to be achieved and formed with a plurality of loops disposed in mutual side-by-side relation.
The invention also relates to a toothed belt of elastomeric material of the type comprising: at least a layer of reinforcing cords of annular configuration; at least a layer of elastomeric material incorporating said reinforcing layer and exhibiting, on an inner surface thereof, a plurality of teeth extending transversely to the circumferential extension or length of the belt and spaced apart from each other by the same distance, according to a predetermined toothing pitch.
2. Prior Art
It is known that toothed belts of elastomeric material consist of a body closed in the form of a loop and are essentially comprised of a layer of longitudinal reinforcing cords incorporated within at least a layer of elastomeric material in which a plurality of shaped teeth, spaced apart from each other by the same distance according to a predetermined toothing pitch, are defined on the inner side of the circumferential belt extension.
The manufacture of these belts involves producing a tubular sleeve having inner and outer circumferential surfaces which is obtained by successively wrapping the layer of reinforcing cords and the layer of elastomeric material in the raw state over a cylindrical drum having axially extending grooves or tooth spaces matching the shape of the toothing to be achieved.
The sleeve disposed on the drum is then submitted to a simultaneous molding and cross-linking process within an autoclave. In particular, the sleeve is submitted to a homogeneously applied pressure for a predetermined period of time. Thereafter the elastomeric material is urged into the axially extending grooves of the drum in order to cause the formation of teeth.
During this process, the overall processing time of which is not generally less than 15 minutes, the sleeve is submitted to high temperatures, generally in the range of 150.degree. to 170.degree. C., in order to cause the complete cross-linking of the elastomeric material and, consequently, the structural stabilization of the obtained toothed sleeve.
As far as mass production is concerned, the circumferential extension or length of the drum on which the sleeve is formed is normally coincident with the circumferential extension or length that the finished belts must have. In such cases it is sufficient to slit the sleeve according to several suitably axially spaced circumferentially-parallel lines in order to obtain a plurality of belts having the desired circumferential extension and axial width.
There are, however, many other cases in which the circumferential extension of the drum, and consequently that of the obtained sleeve, is smaller than the circumferential extension that the finished belts must have.
This occurs for example when it is necessary to produce belts of a given circumferential extension in relatively small supplies, which does not justify preparing drums of sizes selected for purposes of producing such belts. In other cases the circumferential extension of required belts may be so great that making drums of very large sizes is involved, so large in fact that they cannot even be introduced into available factory equipment, such as for example vulcanization and molding autoclaves, and grinding and packaging machines.
In order to solve such problems, the sleeve obtained from the molding and cross-linking process is slit according to a line of helical extension, the pitch of which is coincident with the width of the belts to be achieved. In this way a semifinished product in the form of a continuous strip is produced which lends itself to be cut to size according to the circumferential extension of the desired belt which is thus obtained by joining end-to-end the opposite ends of the strip length cut to size.
By this method it is possible to produce toothed belts of any circumferential extension, but it is necessary to accept the unavoidable structural weakening of the belt caused by the end-to-end junction present on the circumferential extension thereof.
U.S. Pat. No. 4,083,838 discloses a method by which it is possible to obtain belts of thermoplastic material in the desired length without end-to-end junctions of the above described type being required.
In accordance with this method, a semifinished product in the form of a continuous strip, on which a toothing in its final configuration has been previously produced, is passed over two rollers having parallel axes, mutually positioned according to a distance between centers corresponding to the desired circumferential extension to be given to the obtained belt. The wrapping of the semifinished product is carried out helically according to a pitch corresponding to the width of the semifinished product so as to form a plurality of loops disposed consecutively in side-by-side relation and mating at the respective opposite edges.
A heating means is provided close to one of the rollers on which the wrapping is carried out, said means, assisted by a presser ribbon guided according to a determined wrapping arc about the roller, causing the plasticization of the thermoplastic material forming the semifinished product, so that the loops disposed in side-by-side relation are firmly bonded to each other.
Thus an annular toothed element is produced that lends itself to be cut according to circumferential lines thereby giving rise to a plurality of belts having the desired width and circumferential extension.
Presently it is impossible to use the above method to make toothed belts in which elastomeric material is used in place of the thermoplastic material.
In fact, cross-linking carried out in the autoclave before the semifinished product takes the form of a continuous belt would make the subsequent melting of the elastomeric material for obtaining the mutual junction between contiguous loops an impossible achievement.
In accordance with his own experience, the Applicant has conceived of lowering the temperature and residence times of the manufactured article in the autoclave, in order to avoid cross-linking of the elastomeric material, which operation would have been executed in a subsequent step; however, it has been found that the formation of teeth on the inner surface of the sleeve did not take place in a regular manner, so that it was impossible to resort to this procedure.
In accordance with the present invention it has been found that in order to obtain toothed belts made of elastomeric material, of any circumferential extension without mechanical junctions, it is possible to resort to a process in which the above autoclaving step is used to fundamentally form the toothing pitch without cross-linking the elastomeric material. That is, intertooth spaces and teeth, the configurations of which only partly correspond to the final configuration of the teeth are first produced, and, in a subsequent step, cross-linking of grooves (and/or projections) is conducted by placing the noncross-linked product between two mold members, and pressing the members together in the presence of heat to give the teeth their ultimate shape and, by cross-linking of the elastomeric material, causing the contiguous loops of the annular element to adhere to each other so as to form a belt of the desired length devoid of any mechanical junctions. In practice, a process is adopted in which a semifinished product, in the form of a continuous strip, is obtained by helically cutting a sleeve that has been previously subjected in an autoclave to a preforming step which is not necessarily intended to complete construction of teeth but does form teeth and intertooth spaces or grooves according to the toothing pitch to be given to the belts, without, however, cross-linking the elastomeric material present in the sleeve. The semifinished product thus obtained in the form of a continuous strip is then helically wrapped about two spaced apart rollers, which are spaced by a distance depending on the desired belt extension or length, in order to produce an endless ring the adjacent loops of which, as they are in the raw state, enable the cross-linking of the elastomeric material to take place after the ring has been cut to the desired width, which results in the production of a junction-free belt. Intertooth spaces or grooves, obtained in the preforming operation are then used to true up the annular element obtained from the strip wrapping, on a matrix employed for molding and cross-linking the final toothing profile.
The present invention in one main aspect thereof relates to a process for manufacturing toothed belts of elastomeric material comprising the following steps: making a sleeve having circumferentially extending inner and outer surfaces, the inner such surface having intertooth spaces and teeth; cutting the sleeve according to a cutting line of helical extension so as to obtain a semifinished product in the form of a continuous strip; wrapping said semifinished product in the form of a continuous strip over two spaced apart wrapping toothed rollers having axes parallel to one another and positioned according to a predetermined distance measured from the centers of said axes, said wrapping being carried out at a pitch corresponding to the width of the semifinished product in order to obtain an annular element of the desired circumferential extension having at least the same width as the toothed belt to be produced and formed with a plurality of loops disposed in mutual side by side relation, characterized by:
a) controlling the temperature and time of the sleeve-making step so as to obtain a plurality of alternate intertooth spaces or grooves and teeth to preform the teeth defining the belt pitch while avoiding cross-linking the elastomeric material of the sleeve;
b) trueing up the annular element in the raw state, by means of said intertooth spaces or teeth positioned between a molding matrix and an opposing mold member;
c) pressing the matrix and the mold toward each other to compress the sleeve and applying heat thereto to vulcanize the sleeve, so as to mold the teeth to the final shape and cause the mutual bonding of the side-by-side loops of the annular element in the raw state, thereby giving rise to a belt of desired length, without mechanical junctions.
More particularly, this process is characterized in that the production of the semifinished product in the form of a continuous strip takes place through the following steps: a cylindrical tubular sleeve is manufactured by wrapping at least a layer of textile reinforcing fibers and at least a layer of raw elastomeric material over a preforming drum with substantially longitudinal teeth circumferentially distributed according to a predetermined toothing pitch; preforming the tubular sleeve by compressing the elastomeric material against the preforming drum teeth so as to produce, on the inner circumferential surface of said sleeve, a plurality of substantially longitudinal grooves distributed according to said toothing pitch, associated with tooth preforming projections; cutting the tubular sleeve according to a cutting line of helical extension, so as to obtain said semifinished product in the form of a continuous strip of predetermined width having transverse grooves distributed according to said toothing pitch, after said wrapping step the annular element provided with said transverse hollows on an inner surface thereof being submitted to the steps of: pressing the annular element between a molding matrix and an opposing mold member which matrix and member act on the inner surface and an outer surface respectively of the annular element so as to define on said inner surface a plurality of shaped teeth spaced apart by transverse grooves; cross-linking the elastomeric material present in the annular element by administering heat thereto, in order to cause the mutual bonding of said loops disposed in side-by-side relation during the wrapping step, as well as a structural stabilization of the obtained toothed belt.
Advantageously, during said pressing step a final mutual trueing of the respectively aligned transverse grooves belonging to the individual loops of the semifinished product previously wrapped in the form of a strip, is carried out.
During said wrapping step a mutual alignment of the transverse grooves present in the individual loops of said semifinished product in the form of a strip is also preferably executed. This alignment is obtained by engagement of said grooves on longitudinal-alignment teeth provided on said wrapping rollers, which are spaced apart by the same distance as said toothing pitch.
Preferably, in the production of the cylindrical tubular sleeve a coating fabric is first placed on the preforming drum before carrying out the wrapping of the textile fiber layer and the raw elastomeric material.
In addition, during said pressing step, a covering fabric previously interposed between said molding matrix and the annular element may also be fastened to the inner surface of the annular element. The application of a covering fabric to the inner surface of the annular element can be exploited in order to execute a presetting of the loops of the semifinished product in the form of a strip before the pressing step.
In this case said covering fabric is applied during said wrapping step by positioning the covering fabric between the wrapping rollers and the annular element being worked.
Alternatively, the mutual presetting of the loops formed with said semifinished product can take place through application, after said wrapping step, of an outer coating sheet to the outer surface of the annular element wrapped around said wrapping rollers.
The cross-linking step of the elastomeric material is preferably obtained simultaneously with the pressing step, through heating of the molding matrix and opposite mold member. In addition, the pressing step is carried out by successive stages to each of which the pressing of a portion of predetermined length of said annular element corresponds.
After the cross-linking step, the edges of the obtained toothed belt may be trimmed.
In the case in which the annular element has an important width, it is possible to execute a circumferential cutting step after said cross-linking step, in order to obtain a plurality of toothed belts of predetermined width.
Alternatively, this circumferential cutting step can be executed before the pressing step.
In accordance with a further feature of the invention, the preforming step is conducted so that the grooves on the sleeve exhibit a depth corresponding to at least 1/3 of the height detectable on the individual teeth of the finished belt, and alternate with projections the transverse preforming outline of which has a surface extension equal to at least 1/2 of the surface extension detectable on the tooth profile of the finished belt.
Preferably, in accordance with the process of the invention, after the molding step at least 10% of said projections should be only partly formed as regards their detectable height and/or their sizes as above described.
More preferably at least 30% of the projections are partly formed as above stated and still more preferably at least 50% of the projections are partly formed as above stated.
Advantageously the application of centripetal pressure during the forming step takes place concurrently with the administration of heat to the sleeve being worked.
In greater detail, the application of centripetal pressure during the preforming step is carried out in an autoclave concurrently with the administration of heat to the sleeve over a period of time determined to be between 1/6 and 1/2 of the time necessary to cause the complete cross-linking of the elastomeric material at the temperature conditions according to which the administration of heat occurs.
Alternatively, the administration of heat to the sleeve is carried out by submitting the latter to temperatures of a value included between 1/2 and 4/5 of the value needed to obtain the complete cross-linking of the elastomeric material over a period of time corresponding to the duration of the administration of heat.
In a preferred embodiment, the application of pressure during a preforming step occurs at a value in a range of 10 bar to 22 bar, over a period of time of 2 to 15 minutes, while administering heat to the sleeve at a temperature in the range 50.degree. C. to 205.degree. C. at the outside thereof, and in the range of 100.degree. C. to 140.degree. C. at the inside thereof, depending on fluid used (that is air, steam or pressurized water).
After the preforming step the material has not been subjected to a vulcanization process and, hence, physical parameters of the vulcanized item such as hardness and ultimate tensile strength are not measurable.
The invention in another aspect thereof also relates to a toothed belt of elastomeric material obtained by the above process, said belt being characterized in that said reinforcing cord layer, in the form of a continuous strip, is helically wrapped in several loops disposed in a side-by-side relation and mated at the respective opposite edges.
Preferably, the belt further comprises at least a coating fabric applied to the inner surface thereof provided with said teeth, said coating fabric being structured in the form of a continuous strip wrapped in several side-by-side loops mating at the respective opposite edges.
The belt may also comprise at least a covering fabric, which is applied to the inner surface thereof provided with teeth, said covering fabric being structured in the form of a continuous strip of the same width as the belt .